[PATCH net v2 1/2] net: ipv4: fix incorrect MTU in broadcast routes

[PATCH net v2 1/2] net: ipv4: fix incorrect MTU in broadcast routes

[Oscar Maes]

Currently, __mkroute_output overrules the MTU value configured for
broadcast routes.

This buggy behaviour can be reproduced with:

ip link set dev eth1 mtu 9000
ip route del broadcast 192.168.0.255 dev eth1 proto kernel scope link src 192.168.0.2
ip route add broadcast 192.168.0.255 dev eth1 proto kernel scope link src 192.168.0.2 mtu 1500

The maximum packet size should be 1500, but it is actually 8000:

ping -b 192.168.0.255 -s 8000

Fix __mkroute_output to allow MTU values to be configured for
for broadcast routes (to support a mixed-MTU local-area-network).

Signed-off-by: Oscar Maes <oscmaes92@gmail.com>
---
 net/ipv4/route.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index fccb05fb3..a2a3b6482 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2585,7 +2585,6 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
 	do_cache = true;
 	if (type == RTN_BROADCAST) {
 		flags |= RTCF_BROADCAST | RTCF_LOCAL;
-		fi = NULL;
 	} else if (type == RTN_MULTICAST) {
 		flags |= RTCF_MULTICAST | RTCF_LOCAL;
 		if (!ip_check_mc_rcu(in_dev, fl4->daddr, fl4->saddr,
-- 
2.39.5

[PATCH net v2 2/2] selftests: net: add test for variable PMTU in broadcast routes

[Oscar Maes]

Added a test for variable PMTU in broadcast routes.

This test uses iputils' ping and attempts to send a ping between
two peers, which should result in a regular echo reply.

This test will fail when the receiving peer does not receive the echo
request due to a lack of packet fragmentation.

Signed-off-by: Oscar Maes <oscmaes92@gmail.com>
---
 tools/testing/selftests/net/Makefile          |  1 +
 tools/testing/selftests/net/broadcast_pmtu.sh | 47 +++++++++++++++++++
 2 files changed, 48 insertions(+)
 create mode 100755 tools/testing/selftests/net/broadcast_pmtu.sh

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index 332f38761..f4aa94588 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -112,6 +112,7 @@ TEST_PROGS += skf_net_off.sh
 TEST_GEN_FILES += skf_net_off
 TEST_GEN_FILES += tfo
 TEST_PROGS += tfo_passive.sh
+TEST_PROGS += broadcast_pmtu.sh
 
 # YNL files, must be before "include ..lib.mk"
 YNL_GEN_FILES := busy_poller netlink-dumps
diff --git a/tools/testing/selftests/net/broadcast_pmtu.sh b/tools/testing/selftests/net/broadcast_pmtu.sh
new file mode 100755
index 000000000..726eb5d25
--- /dev/null
+++ b/tools/testing/selftests/net/broadcast_pmtu.sh
@@ -0,0 +1,47 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Ensures broadcast route MTU is respected
+
+CLIENT_NS=$(mktemp -u client-XXXXXXXX)
+CLIENT_IP4="192.168.0.1/24"
+CLIENT_BROADCAST_ADDRESS="192.168.0.255"
+
+SERVER_NS=$(mktemp -u server-XXXXXXXX)
+SERVER_IP4="192.168.0.2/24"
+
+setup() {
+	ip netns add "${CLIENT_NS}"
+	ip netns add "${SERVER_NS}"
+
+	ip -net "${SERVER_NS}" link add link1 type veth peer name link0 netns "${CLIENT_NS}"
+
+	ip -net "${CLIENT_NS}" link set link0 up
+	ip -net "${CLIENT_NS}" link set link0 mtu 9000
+	ip -net "${CLIENT_NS}" addr add "${CLIENT_IP4}" dev link0
+
+	ip -net "${SERVER_NS}" link set link1 up
+	ip -net "${SERVER_NS}" link set link1 mtu 1500
+	ip -net "${SERVER_NS}" addr add "${SERVER_IP4}" dev link1
+
+	read -r -a CLIENT_BROADCAST_ENTRY <<< "$(ip -net "${CLIENT_NS}" route show table local type broadcast)"
+	ip -net "${CLIENT_NS}" route del "${CLIENT_BROADCAST_ENTRY[@]}"
+	ip -net "${CLIENT_NS}" route add "${CLIENT_BROADCAST_ENTRY[@]}" mtu 1500
+
+	ip net exec "${SERVER_NS}" sysctl -wq net.ipv4.icmp_echo_ignore_broadcasts=0
+}
+
+cleanup() {
+	ip -net "${SERVER_NS}" link del link1
+	ip netns del "${CLIENT_NS}"
+	ip netns del "${SERVER_NS}"
+}
+
+trap cleanup EXIT
+
+setup &&
+	echo "Testing for broadcast route MTU" &&
+	ip net exec "${CLIENT_NS}" ping -f -M want -q -c 1 -s 8000 -w 1 -b "${CLIENT_BROADCAST_ADDRESS}" > /dev/null 2>&1
+
+exit $?
+
-- 
2.39.5

Re: [PATCH net v2 1/2] net: ipv4: fix incorrect MTU in broadcast routes

[kernel test robot]

Hi,

Thanks for your patch.

FYI: kernel test robot notices the stable kernel rule is not satisfied.

The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html#option-1

Rule: add the tag "Cc: stable@vger.kernel.org" in the sign-off area to have the patch automatically included in the stable tree.
Subject: [PATCH net v2 1/2] net: ipv4: fix incorrect MTU in broadcast routes
Link: https://lore.kernel.org/stable/20250703152838.2993-1-oscmaes92%40gmail.com

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH net v2 1/2] net: ipv4: fix incorrect MTU in broadcast routes

[Jakub Kicinski]

On Thu,  3 Jul 2025 17:28:37 +0200 Oscar Maes wrote:
>  	if (type == RTN_BROADCAST) {
>  		flags |= RTCF_BROADCAST | RTCF_LOCAL;
> -		fi = NULL;
>  	} else if (type == RTN_MULTICAST) {
>  		flags |= RTCF_MULTICAST | RTCF_LOCAL;
>  		if (!ip_check_mc_rcu(in_dev, fl4->daddr, fl4->saddr,

Not super familiar with this code, but do we not need to set 
do_cache = false; ? I'm guessing cache interactions may have
been the reason fib_info was originally cleared, not sure if
that's still relevant..

I'd also target this at net-next, unless you can pinpoint
some kernel version where MTU on bcast routes worked..
-- 
pw-bot: cr

Re: [PATCH net v2 1/2] net: ipv4: fix incorrect MTU in broadcast routes

[Oscar Maes]

On Tue, Jul 08, 2025 at 06:54:30PM -0700, Jakub Kicinski wrote:
> On Thu,  3 Jul 2025 17:28:37 +0200 Oscar Maes wrote:
> >  	if (type == RTN_BROADCAST) {
> >  		flags |= RTCF_BROADCAST | RTCF_LOCAL;
> > -		fi = NULL;
> >  	} else if (type == RTN_MULTICAST) {
> >  		flags |= RTCF_MULTICAST | RTCF_LOCAL;
> >  		if (!ip_check_mc_rcu(in_dev, fl4->daddr, fl4->saddr,
> 
> Not super familiar with this code, but do we not need to set 
> do_cache = false; ? I'm guessing cache interactions may have
> been the reason fib_info was originally cleared, not sure if
> that's still relevant..
> 
> I'd also target this at net-next, unless you can pinpoint
> some kernel version where MTU on bcast routes worked..
> -- 
> pw-bot: cr

The caching mechanism was introduced after this line, back when nhc was embedded in fib_info.
(see https://lore.kernel.org/netdev/20120720.142612.691540831359186107.davem@davemloft.net/)

I'll resend to net-next.